Multi-function spectrometer-on-chip with a single detector array

ABSTRACT

Various embodiments of apparatuses, systems and methods are described herein for a spectrometer comprising at least two dispersive elements configured to receive at least one input optical signal and generate two or more pluralities of spatially separated spectral components, at least a portion of the at least two dispersive elements being implemented on a first substrate; and a single detector array coupled to the at least two dispersive elements and configured to receive and measure two or more pluralities of narrowband optical signals derived from the two or more pluralities of spatially separated spectral components, respectively.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.14/035,348, filed on Sep. 24, 2013 and now issued to patent as U.S. Pat.No. 8,937,717 which claims the benefit of U.S. Provisional ApplicationNo. 61/704,911 filed on Sep. 24, 2012. The entire contents ofapplication Ser. No. 14/035,348 and 61/704,911 are hereby incorporatedby reference.

FIELD

The various embodiments described herein generally relate to anapparatus and method for implementing and using a spectrometer thatperforms multiple functions.

BACKGROUND

An optical spectrometer is a system that is used to sample the spectralcomponents of an optical signal. In a general case, dispersivespectrometers use a dispersive element such as a diffraction grating tospatially distribute the spectral components of the optical signal.These spectral components are then measured by a linear array ofdetector elements, also known as a detector array.

Spectrometers that are built using discrete free-space opticalcomponents such as gratings and lenses are typically designed to measureonly the wavelength content of one optical signal in one way, forexample with a characteristic dispersion over a fixed operatingbandwidth. In one example, these conventional spectrometers aretypically not designed to measure both the spectrum and the polarizationstate of light in the optical signal. Polarization describes theorientation of the oscillation of the light wave's electric field. Thepolarization of light can be described using a combination of twoorthogonal basis states. While there are some spectrometers that splitthe light signal into two polarization states and measure the spectralcomponents for each polarization, these spectrometers use more opticalcomponents and/or require additional signal processing. As a result, thepolarization needs to be controlled and managed at each interactionbetween a free-space beam and an optical surface, and this is generallya costly and difficult task using free-space optics.

SUMMARY OF VARIOUS EMBODIMENTS

In one broad aspect, at least one embodiment described herein provides aspectrometer comprising at least two dispersive elements configured toreceive at least one input optical signal and generate two or morepluralities of spatially separated spectral components, at least aportion of the at least two dispersive elements being implemented on afirst substrate; and a single detector array coupled to the at least twodispersive elements and configured to receive and measure two or morepluralities of narrowband optical signals derived from the two or morepluralities of spatially separated spectral components, respectively.

In at least some embodiments, the at least two dispersive elementscomprise a first dispersive element disposed on the first substrate andconfigured to receive a first input optical signal and generate a firstplurality of spatially separated spectral components; and a seconddispersive element disposed on the first substrate and configured toreceive a second input optical signal and generate a second plurality ofspatially separated spectral components.

In at least some embodiments, the spectrometer comprises a polarizationsplitter configured to receive an initial input optical signal andspatially separate the initial input optical signal into the first inputoptical signal having the first polarization state and the second inputoptical signal having the second polarization state over the operatingbandwidth range. The first dispersive element may be coupled to thepolarization splitter to receive the first input optical signal andgenerate the first plurality of spatially separated spectral componentshaving the first polarization state; and the second dispersive elementmay be coupled to the polarization splitter to receive the second inputoptical signal and generate the second plurality of spatially separatedspectral components having the second polarization state.

In at least some embodiments, the polarization splitter may beimplemented on one of the first substrate or a separate substrate.

In at least some embodiments, the at least two dispersive elements areconfigured to share a common footprint in a polarization-splittinggrating configuration implemented on the first substrate, thepolarization-splitting grating configuration having a difference ineffective indices of refraction for first and second polarization statesover an operating bandwidth range, wherein the polarization-splittinggrating configuration is configured to receive an initial input opticalsignal as the at least one input optical signal and spatially separatethe initial input optical signal into a first plurality of spatiallyseparated signals having a first polarization state and a secondplurality of spatially separated signals having a second polarizationstate.

In at least some embodiments, a difference in effective indices ofrefraction for the first and second polarization states is large enoughto spatially separate the two polarization components of the initialinput optical signal such that a minimum output angle of the firstplurality of spatially separated spectral components having the firstpolarization state is larger than a maximum output angle of the secondplurality of spatially separated spectral components having the secondpolarization state over the operating bandwidth range.

In at least some embodiments, the polarization-splitting gratingconfiguration comprises a Planar Concave Grating (PCG)polarization-splitting grating having a difference in effective indicesof refraction for the first and second polarization states such that thefirst plurality of spatially separated spectral components having thefirst polarization state is disposed along a first portion of an outputof the PCG polarization-splitting grating and the second plurality ofspatially separated spectral components having the second polarizationstate is disposed along a second portion of the output different thanthe first portion.

In at least some embodiments, the first plurality of spatially separatedspectral components having the first polarization state may be disposedalong a first portion of an output focal curve of the PCGpolarization-splitting grating and the second plurality of spatiallyseparated spectral components having the second polarization state maybe disposed along a second portion of the output focal curve differentthan the first portion.

In at least some embodiments, the polarization-splitting gratingconfiguration comprises an Arrayed Waveguide Grating (AWG) gratingdispersive element having a difference in effective indices for thefirst and second polarization states such that the first plurality ofspatially separated spectral components having the first polarizationstate is disposed along a first portion of an output of the AWGpolarization-splitting grating and the second plurality of spatiallyseparated spectral components having the second polarization state isdisposed along a second portion of the output different than the firstportion.

In at least some embodiments, the first plurality of spatially separatedspectral components having the first polarization state may be disposedalong a first portion of an output focal curve of the AWGpolarization-splitting grating and the second plurality of spatiallyseparated spectral components having the second polarization state maybe disposed along a second portion of the output focal curve differentthan the first portion.

In at least some embodiments, the first and second plurality ofspatially separated spectral components are transmitted using gratingmode orders m₁ and m₂ and the first and second effective indices ofrefraction (n_(eff,1) and n_(eff,2) respectively) of thepolarization-splitting grating configuration are designed to satisfy thecondition m₁λ_(min)/n_(eff,1)(λ_(min))>m₂λ_(max)/n_(eff,2)(λ_(max)),where numerals 1 and 2 represent the first and second polarizationstates respectively, m₁ and m₂ can be similar or different mode orders,λ_(min) and λ_(max) define minimum and maximum wavelengths over theoperating bandwidth range and n_(eff,1)(λ_(min)) and n_(eff,2)(λ_(max))represent effective indices of refraction at the minimum and maximumwavelengths of the operating bandwidth range.

In at least some embodiments, the polarization-splitting gratingconfiguration is further configured to independently focus the first andsecond pluralities of spatially separated spectral components havingdifferent polarization states by positioning each facet in a set offacets to substantially simultaneously provide an optical path lengthdifference of m₂λ₀/n_(eff,2)(λ₀) and m₁λ₀/n_(eff,)1/(λ₀) compared toadjacent facets, where numerals 1 and 2 represent the first and secondpolarization states respectively, m₁ and m₂ are similar or differentmode orders, λ₀ is a central wavelength for a given mode order andn_(eff,1)(λ₀) and n_(eff,2)(λ₀) represent the indices of refraction forthe first and second mode orders respectively.

In at least some embodiments, the spectrometer further comprises two ormore waveguide arrays to capture and transmit the two or morepluralities of narrowband optical signals from outputs of the at leasttwo dispersive elements to inputs of the single detector array.

In at least some embodiments, the single detector array may be on asecond substrate of a different chip and an edge of the first substratemay be directed to a face of the second substrate in order to capturethe two or more pluralities of narrowband optical signals at inputs ofthe single detector array.

In at least some embodiments, the single detector array may be on asecond substrate of a different chip and a face of the first substratemay be directed to a face of the second substrate in order to capturethe two or more pluralities of narrowband optical signals at inputs ofthe single detector array.

In at least some embodiments, the single detector array may be locatedon the first substrate.

The various embodiments of the spectrometers described herein maygenerally comprise readout electronics to receive the measurements fromthe single detector array and generate output samples therefrom having adesired output data format.

In at least some embodiments, the readout electronics may be located onthe first substrate.

In at least some embodiments, the single detector array may be locatedon a different substrate than the first substrate and one or more lensesmay be located between the first substrate and the single detector arrayto refocus the two or more pluralities of narrowband optical signals andto improve coupling efficiency.

In another broad aspect, at least one embodiment described hereinprovides a method of measuring two or more pluralities of spatiallyseparated spectral components, the method comprising receiving at leastone input optical signal; generating two or more pluralities ofspatially separated spectral components from the at least one inputoptical signal by providing the at least one input optical signal to atleast two dispersive elements where at least a portion of the at leasttwo dispersive elements is implemented on a first substrate; derivingtwo or more pluralities of narrowband optical signals from the two ormore pluralities of spatially separated spectral components,respectively; and measuring the two or more pluralities of spatiallyseparated spectral components with a single detector array.

In at least some embodiments, the at least two dispersive elementscomprise first and second dispersive elements and the method furthercomprises receiving a first input optical signal at the first dispersiveelement disposed on the first substrate; generating a first plurality ofspatially separated spectral components with the first dispersiveelement; receiving a second input optical signal at the seconddispersive element disposed on the first substrate; and generating asecond plurality of spatially separated spectral components with thesecond dispersive element.

In at least some embodiments, the method further comprises receiving aninitial input optical signal as the at least one input optical signal ata polarization splitter; generating the first input optical signal tohave a first polarization state over an operating bandwidth range usingthe polarization splitter; generating the second input optical signal tohave the second polarization state over the operating bandwidth rangeusing the polarization splitter; generating the first plurality ofspatially separated spectral components having the first polarizationstate with the first dispersive element; and generating the secondplurality of spatially separated spectral components having the secondpolarization state with the second dispersive element.

In at least some embodiments, the at least two dispersive elements areconfigured to share a common footprint in a polarization-splittinggrating configuration implemented on the first substrate, thepolarization-splitting grating configuration having a difference ineffective indices of refraction for first and second polarization statesover an operating bandwidth range and the method comprises receiving aninitial input optical signal as the at least one input optical signal atthe polarization-splitting grating configuration; and spatiallyseparating the initial input optical signal into a first plurality ofspatially separated signals having a first polarization state and asecond plurality of spatially separated signals having a secondpolarization state.

In at least some embodiments, the method comprises spatially separatingthe two polarization states of the initial input optical signal suchthat a minimum output angle of the first plurality of spatiallyseparated spectral components having the first polarization state islarger than a maximum output angle of the second plurality of spatiallyseparated spectral components having the second polarization state overthe operating bandwidth range.

In at least some embodiments, the polarization-splitting gratingconfiguration has a difference in effective indices of refraction forthe first and second polarization states such that the first pluralityof spatially separated spectral components having the first polarizationstate is disposed along a first portion of an output of thepolarization-splitting grating configuration and the second plurality ofspatially separated spectral components having the second polarizationstate is disposed along a second portion of the output different thanthe first portion.

In at least some embodiments, the first plurality of spatially separatedspectral components having the first polarization state may be disposedalong a first portion of an output focal curve of thepolarization-splitting grating configuration and the second plurality ofspatially separated spectral components having the second polarizationstate may be disposed along a second portion of the output focal curvedifferent than the first portion.

In yet another broad aspect, at least one embodiment described hereinprovides a spectrometer for measuring spectral components of at leastone input optical signal, wherein the spectrometer comprises at leastone dispersive element configuration adapted to spatially separate atleast a first portion of the at least one input optical signal into afirst plurality of spatially separated spectral components having afirst polarization state and at least a second portion of the at leastone input optical signal into a second plurality of spatially separatedspectral components having a second polarization state, the at least onedispersive element configuration being adapted to operate over anoperating bandwidth range with at least a portion of the at least onedispersive element configuration being implemented on a first substrate;and a detector array coupled to the at least one dispersive elementconfiguration and configured to receive and measure data related to afirst and a second plurality of narrowband optical signals derived fromthe first and second plurality of spatially separated spectralcomponents, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the various embodiments described herein,and to show more clearly how these various embodiments may be carriedinto effect, reference will be made, by way of example, to theaccompanying drawings which show at least one example embodiment, and inwhich:

FIG. 1 is a block diagram of an example embodiment of apolarization-sensitive spectrometer;

FIG. 2 is a block diagram of another example embodiment of apolarization-sensitive spectrometer;

FIG. 3 is a block diagram of another example embodiment of apolarization-sensitive spectrometer;

FIG. 4A is a diagram of a polarization-splitting grating configurationthat can provide both dispersion and polarization-splitting functions;

FIG. 4B is a schematic of a Planar Concave Grating (PCG)polarization-splitting grating that provides both dispersion andpolarization-splitting functions;

FIG. 4C is a schematic of an Arrayed Waveguide Grating (AWG)polarization-splitting grating that provides both dispersion andpolarization-splitting functions;

FIGS. 5A-5D are diagrams illustrating the birefrengence versuswavelength and bandwidth as well as the relation between bandwidth andwavelength for a polarization-sensitive spectrometer that uses anexample embodiment of a PCG polarization-splitting grating to provideboth dispersion and polarization-splitting functions;

FIG. 6 illustrates a scenario of incomplete polarization-splitting whenthe bandwidth of the input optical signal is larger than the maximumbandwidth of a polarization-splitting grating configuration in whichcase a secondary polarization-splitting element or wavelength-splittingelement can be used for complete separation of the polarizationcomponents of the input optical signal;

FIG. 7A shows an example embodiment of an arrangement between aspectrometer on one chip and a detector array on a different chip;

FIG. 7B shows another example embodiment of an arrangement between aspectrometer on one chip and a detector array on a different chip; and

FIG. 8 shows an example embodiment of a spectrometer having multipledispersive elements on a planar substrate and a single detector array.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Various apparatuses or processes will be described below to provide anexample of an embodiment of the each claimed subject matter. Noembodiment described below limits any claimed subject matter and anyclaimed subject matter may cover processes or apparatuses that differfrom those described below. The claimed subject matter are not limitedto apparatuses or processes having all of the features of any oneapparatus or process described below or to features common to multipleor all of the apparatuses or processes described below. It is possiblethat an apparatus or process described below is not an embodiment of anyclaimed subject matter. Any subject matter disclosed in an apparatus orprocess described herein that is not claimed in this document may be thesubject matter of another protective instrument, for example, acontinuing patent application, and the applicants, inventors or ownersdo not intend to abandon, disclaim or dedicate to the public any suchsubject matter by its disclosure in this document.

Furthermore, it will be appreciated that for simplicity and clarity ofillustration, where considered appropriate, reference numerals may berepeated among the figures to indicate corresponding or analogouselements. In addition, numerous specific details are set forth in orderto provide a thorough understanding of the embodiments described herein.However, it will be understood by those of ordinary skill in the artthat the embodiments described herein may be practiced without thesespecific details. In other instances, well-known methods, procedures andcomponents have not been described in detail so as not to obscure thedescription of the embodiments presented herein. Also, the descriptionis not to be considered as limiting the scope of the embodimentsdescribed herein in any way, but rather as merely describing theimplementation of various embodiments as described.

It should be noted that the terms or phrases “an embodiment,”“embodiment,” “embodiments,” “the embodiment”, “the embodiments”, “oneor more embodiments”, “some embodiments”, “at least one embodiment”, “atleast some embodiments” and “one embodiment” mean “one or more (but notall) embodiments of the present subject matter”, unless expresslyspecified otherwise.

It should also be noted that the terms “including”, “comprising” andvariations thereof mean “including but not limited to”, unless expresslyspecified otherwise. A listing of items does not imply that any or allof the items are mutually exclusive, unless expressly specifiedotherwise.

It should also be noted that the terms coupled or coupling as usedherein can have several different meanings depending in the context inwhich the term is used. For example, the terms coupled or coupling canhave at least one of a mechanical, electrical or optical, connotation.For example, depending on the context, the terms coupled or coupling mayindicate that two elements or devices can be at least one of physically,electrically or optically directly connected to one another or they maybe linked to one another through one or more intermediate elements ordevices via at least one of a physical, an electrical or an opticalconnection such as, but not limited to a wire, a fiber optic cable or awaveguide, for example.

It should be noted that terms of degree such as “substantially”, “about”and “approximately” as used herein mean a reasonable amount of deviationof the modified term such that the end result is not significantlychanged. These terms of degree should be construed as including adeviation of up to a certain amount of the modified term if thisdeviation would not negate the meaning of the term it modifies.

Furthermore, the recitation of numerical ranges by endpoints hereinincludes all numbers and fractions subsumed within that range (e.g. 1 to5 includes 1, 1.5, 2, 2.75, 3, 3.90, 4, and 5). It is also to beunderstood that all numbers and fractions thereof are presumed to bemodified by the term “about.” The term “about” means a deviation of upto plus or minus a certain amount of the number to which reference isbeing made without negating the meaning of the term that it modifies.

Furthermore, in the following passages, different aspects of theembodiments are defined in more detail. Each aspect so defined may becombined with any other aspect or aspects unless clearly indicated tothe contrary. In particular, any feature indicated as being preferred oradvantageous may be combined with at least one other feature or featuresindicated as being preferred or advantageous.

An optical spectrometer typically receives one input optical signal andmeasures the wavelength content of that optical signal over a particularoperating wavelength range with a particular spectral resolution ordispersion. Conventionally, multiple spectrometers must be constructedin order to measure multiple optical signals, multiple spectralresolutions, multiple operating bandwidth ranges, or multiple opticalcharacteristics besides wavelength such as polarization, for example.The cost and size of a collection of multiple spectrometers tend toincrease linearly, e.g. three spectrometers cost three times as much asone spectrometer and take up three times the space. This scaling isdriven in large part by the requirement for one camera (containing onedetector array plus readout electronics) per spectrometer, plus theassociated data connections and computing requirements to supportmultiple cameras.

This scaling of cost and size prohibits the practical implementation ofmany useful collections of spectrometers. In many cases, it is desirableto create a multi-function spectrometer with a single detector arraywhich can perform multiple functions with little increase in size orcost compared to a single-function spectrometer. For example, in somecases an optical signal may have spectral features at multiplewavelength ranges such that it is desirable to measure each wavelengthrange independently, potentially with a different spectral resolution ineach wavelength range. In some cases, multiple optical signals might becollected from multiple samples or from multiple areas on the samesample, and it is desirable to measure the wavelength content of all theoptical signals at once with the same synchronized trigger event. Instill other cases, it is desirable to measure the wavelength content oftwo optical signals which are output from an interferometer to implementa balanced detection scheme.

Various embodiments according to the teachings described herein havebeen newly discovered that can be used to implement a multi-functionspectrometer with a single detector array.

In some cases, it is desirable to create a polarization-sensitivespectrometer that can independently measure the spectral components ofboth polarizations of an input optical signal. For example,Polarization-Sensitive Optical Coherence Tomography (PS-OCT) is animaging technique that can provide enhanced image contrast betweentissues or materials with different polarization-altering properties.Spectral Domain PS-OCT (SD-PS-OCT) is preferred over Time Domain PS-OCTdue to the speed of data capture. Data capture in SD-PS-OCT can beperformed with a polarization-sensitive spectrometer.

However, conventional SD-PS-OCT systems are typically large andexpensive due to the use of only free-space optic components and theneed for at least one of multiple cameras, multiple beam splitters, andmultiple wave plates, for example, and the costs of accurately aligningand calibrating these systems. For instance, the traditional measurementapproach for SD-PS-OCT includes a polarization beam splitter to separatean input optical signal into two polarization components, followed bytwo independent spectrometers to measure the polarization components inparallel. The requirement for two spectrometers, and in particular twoline-scan cameras, adds significantly to system cost. Accordingly, apolarization-sensitive spectrometer that uses only one camera willreduce costs. Similarly, the incremental cost of adding a secondspectrometer to a chip, as described in some embodiments here, will beconsiderably less than that of acquiring a second free-space opticalspectrometer.

Generally speaking, the polarization of a polarized input optical signalmay be decomposed into any two orthogonal basis states to describe anelliptical polarization. Two specific cases of an ellipticalpolarization are a circular polarization and a linear polarization.Measurement of the polarization state of a polarized input light signalcan be accomplished by measuring the intensities of two orthogonal basisstates, for example using a polarization-sensitive spectrometer. Anincoherent or partially polarized input optical signal may be describedmore completely by the Stokes parameters. Measurement of the Stokesparameters may include multiple manipulations of the state of the inputoptical signal combined with multiple measurements using apolarization-sensitive spectrometer. In the following description aninput optical signal is measured as the combination of two linearlypolarized orthogonal basis states. In other embodiments, otherpolarization states can be measured by a polarization-sensitivespectrometer such as right-hand circularly polarized and left-handcircularly polarized orthogonal basis states, for example.

The challenges with conventional PS-OCT systems that are based onfree-space optical components can be overcome by building simplerpolarization-sensitive spectrometers, as described herein, with a singledetector array or camera by locating two or more dispersive elements ona planar substrate (also referred to as an integrated circuit, a waferor a chip). An optical system that is at least partially implemented ona chip includes planar optical waveguides on the surface of the chip toroute and process optical signals. Integrated optical components are atleast partially sensitive to the polarization of an input opticalsignal. For example, the polarization states of an input optical signalin an integrated waveguide with a substantially rectangularcross-section may be described as being the quasi transverse electric(“TE”) and quasi transverse magnetic (“TM”) polarization states (alsoknown as modes).

In an integrated waveguide, the TE and TM modes have a differenteffective index n_(eff) or propagation speed c/n_(eff) due todifferences in geometry or refractive index between the verticalcross-section and the horizontal cross-section of the waveguide. Thedifference in effective index is referred to as birefringence which canbe represented mathematically as shown in equation 1:Δn _(eff) =n _(eff,TE) −n _(eff,TM)  (1)where Δn_(eff) is the difference in effective index of refraction due tobirefringence, n_(eff,TE) is the effective index of refraction for theTE mode and n_(eff,TM) is the effective index of refraction for the TMmode. Birefringence causes the TE and TM modes to propagate differentlythrough an integrated optical structure. Accordingly, two dispersiveelements can be designed with one each for the TE and TM polarizationstates. In some embodiments, the two dispersive elements can then beoverlaid to utilize the same footprint and the same grating (i.e. thetwo dispersive elements have a common footprint and a common grating).This combination of two dispersive elements is defined and referredherein as a polarization-splitting grating configuration. In some cases,this arrangement may be alternatively referred to as at least onedispersive element configuration.

In at least some of the various embodiments described herein, an on-chipdispersive element, such as an Arrayed Waveguide Grating (AWG) or aPlanar Concave Grating (PCG) for example, is designed such that theTE-polarized portion of light at a given wavelength is directed todifferent output waveguides than the TM-polarized portion of light atthat same wavelength allowing for the simultaneous independentmeasurement of the spectral content of both polarization components ofan input optical signal. As described herein, this can be done bydesigning the birefringence of an on-chip polarization-splitting gratingsuch that the two polarizations of an optical signal are directed to twooutput regions and measuring the polarizations of an optical signal witha single array of detectors.

In addition to splitting the polarizations of an input optical signal,at least some of the spectrometers described herein can also be designedsuch that both polarization components of an input optical signal arewell-focused with little or no aberration. By using at least oneintegrated component, the various embodiments of the spectrometersdescribed herein also reduce the size and cost of SD-PS-OCT systemscompared to conventional systems that are based on free-space opticalcomponents.

The design and implementation of the dispersive element in at least someof the embodiments described herein is in contrast to several instancesof conventional integrated structures that process optical signals sincethese conventional integrated structures are designed to decrease oreliminate the effective birefringence Δn_(eff) between the twopolarization components of an optical signal. In this way, the twopolarization components of the optical signal propagate similarlythrough these conventional integrated structures, and the resultingconventional system is polarization-insensitive.

Another advantage of using integrated optical components is that opticalcomponents located on a chip can be pre-aligned with each other duringthe chip fabrication process, resulting in fewer components that need tobe manually aligned during a final assembly process. In particular,optical waveguides on a spectrometer chip (e.g. a chip having at leastone dispersive element) can be used to rearrange the outputs from one ormore dispersive elements and route them to interface appropriately witha single detector array. Then by aligning the spectrometer chip to adetector array, all of the dispersive element outputs are simultaneouslyaligned with the inputs of the detector elements of the detector arrayin a single manufacturing step. This approach significantly reduces thecosts and effort associated with constructing a system of multiplefree-space optical components which must then be manually aligned, as isdone conventionally.

Furthermore, the robustness of the various embodiments described hereinlend themselves well to mobile and point-of-care applications where OCTand PS-OCT systems have previously not been used.

Another advantage of using integrated optical components is that thepolarization does not need to be controlled at each interaction betweena free-space beam and an optical surface for integrated structures. Infact, in the case of an integrated photonic platform, the waveguidestructures are polarization preserving, offering a degree of simplicityand design flexibility not possible with free-space designs.

In some embodiments, at least some elements are composed of waveguidesformed on a planar substrate. In some embodiments, these waveguides canbe comprised of materials that are transparent in the near infraredspectrum in the ranges typically used in OCT systems, such as, but notlimited to the 850 nm, 1050 nm or 1310 nm spectral bands in someembodiments. However, it should be appreciated that in other embodimentsalternative materials can be chosen that are appropriate for aparticular wavelength or range of wavelengths of light. In some of thevarious embodiments, it can be preferable that the materials used toform waveguides have a high refractive index contrast, such as a core tocladding ratio of 1.05:1 or greater, which can confine light and enablemore compact photonic components as compared to materials having a lowrefractive index contrast.

In some embodiments, waveguides can be comprised of silicon nitride,silicon oxynitride, silicon, SUB, doped glass, other polymers or othersuitable materials.

In some embodiments, the integrated elements of these embodiments can beformed on a planar substrate using photolithography. However, it shouldbe understood that photonic circuits can be fabricated by other methods,such as, but not limited to, electron beam lithography or nanoimprintlithography, for example.

In embodiments where elements are formed on a planar substrate usingphotolithography and where waveguides and other photonic elements on theplanar substrate are made of silicon nitride, a standard silicon wafercan be used having several microns of silicon dioxide thermally grown ona top surface of the substrate as a lower waveguide cladding. In atleast some of the embodiments described herein, a thickness of 3-4microns of silicon dioxide can be used; however, it should be understoodthat other thicknesses can be used and may be appropriately chosen basedon the wavelength range of optical input signals to be analyzed and/orprocessed. In some embodiments, silicon dioxide can be deposited byother techniques such as plasma enhanced chemical vapor deposition. Insome embodiments, a material other than silicon dioxide may be used as alower waveguide cladding.

Silicon nitride can then be deposited onto the planar substrate, and insome embodiments, a few hundred nanometers of stoichiometric siliconnitride can be deposited using low pressure chemical vapor deposition.An anti-reflection coating layer such as, but not limited to, Rohm andHaas AR3 can additionally be applied by spin coating onto the planarsubstrate, which can enhance the performance of the photolithographyprocess. A UV-sensitive photoresist such as, but not limited to, ShipleyUV210 can then be applied by spin coating onto the planar substrate.

The planar substrate can be patterned using a photolithographicpatterning tool at an appropriate exposure to expose the resist with apattern of waveguides and other devices. After being exposed, the planarsubstrate can be developed with MicroChemicals AZ 726MIF or anothersuitable developer to remove unexposed resist. A descum process can beused with a plasma etcher to remove residual resist and the pattern inthe resist can be reflowed, in some embodiments for several minutes,with a hot plate to smooth out any surface roughness.

The silicon nitride on the planar substrate can be etched usinginductively coupled reactive ion etching (ICP RIE) with a CHF₃/O₂recipe. The resist mask used for etching can then be removed in anoxygen plasma or in a resist hot strip bath which contains heatedsolvents.

The planar substrate can be annealed in a furnace oxide tube, in someembodiments at 1,200° C. for three hours. This can tend to reducematerial absorption losses in embodiments where an optical sourcegenerates an optical signal at wavelengths that are near infrared.

The planar substrate can then be covered in oxide, which in someembodiments can be done by using high temperature oxide deposited infurnace tubes or by using plasma enhanced chemical vapour deposition.The planar substrate can then be diced and the end facets can bepolished which can improve coupling of waveguides and other opticalelements formed on the planar substrate. Alternatively, the end facetscan be lithographically defined and etched using a deep reactive-ionetching process such as, but not limited to, the Bosch process, forexample.

It should be noted that there may be variations to the fabricationtechniques described above depending on the particular embodiment of thespectrometer that is being manufactured and/or the particular use of thespectrometer.

Referring now to FIG. 1, shown therein is a block diagram of an exampleembodiment of a polarization-sensitive spectrometer 10. The spectrometer10 comprises an off-chip polarization splitter 12, a first dispersiveelement 14, a second dispersive element 16, a first waveguide array 18,a second waveguide array 20, and a detector array 24 having output lines26 coupled to readout electronics 28. The first and second dispersiveelements 14 and 16 and the first and second waveguide arrays 18 and 20are implemented on a substrate of an integrated circuit or chip 22. Theoutput of the spectrometer 10 can be used to generate a spectrum of aninitial input optical signal 30 as a function of frequency orwavelength.

The initial input optical signal 30 containing one or both of TE and TMpolarization components is input to the spectrometer 10. It should benoted that the term input light signal can also be used for the initialinput optical signal 30. The polarization splitter 12 is used tospatially separate the two polarization components TM and TE of theinitial input optical signal 30. Each of the TM and TE components arethen coupled onto the spectrometer chip 22. The splitter 12 may beimplemented using free-space optics, fiber optics, or a separatephotonic chip. For example, the splitter 12 may be a polarization beamsplitter or a fiber optic polarization splitter.

The first and second dispersive elements 14 and 16 are preferablyoptimized for use with a particular polarization, for example byconsidering the effective index n_(eff), group index n_(g), andwaveguide mode size of the desired polarization. Accordingly, the firstdispersive element 14 generates a plurality of spatially separatedspectral components having TM polarization components representative ofthe spectrum of the TM input optical signal. The second dispersiveelement 16 generates a plurality of spatially separated spectralcomponents having TE polarization components representative of thespectrum of the TE input optical signal. Examples of on-chip dispersiveelements that can be used include, but are not limited to, ArrayedWaveguide Gratings (AWGs) and Planar Concave Gratings (PCGs).

The waveguide array 18 has a plurality of waveguides that are arrangedto capture a plurality of narrowband optical signals having TMpolarization components and route these signals in an appropriate mannerto interface with a first portion of detector pixels (also known asdetector elements) from the detector array 24. For example, the detectorarray 24 typically comprises a linear array of detector elements inwhich case the output ports of the plurality of waveguides of thewaveguide array 18 are arranged with a linear pitch at the edge of thespectrometer chip 22 that interfaces with the detector array 24. In someembodiments, each narrowband signal may be absorbed and detected by asingle detector pixel in the detector array 24. In alternativeembodiments, each narrowband signal may be absorbed and detected bymultiple detector pixels in the detector array 24.

The waveguide array 20 has a plurality of waveguides that are arrangedto capture a plurality of narrowband optical signals having TEpolarization components and route these signals in an appropriate manner(as described for waveguide array 18) to interface with a second portionof detector pixels from the detector array 24 that is different from thefirst portion of detector pixels.

This embodiment holds an advantage over conventional free-space opticsystems because the waveguide arrays 18 and 20 can be precisely alignedto the dispersive elements 14 and 18 during the chip manufacturingprocess, such that aligning the chip 22 to the detector array 24automatically aligns the dispersive elements 14 and 16, the waveguidearrays 18 and 20 and the inputs of the detector array 24. This is notpossible with conventional free-space optic systems.

In an alternative embodiment, the dispersive elements 14 and 16 candirectly illuminate the detector pixels of the detector array 24 withoutthe use of the waveguide arrays 18 and 20, respectively. In this case,the detector elements of the detector array 24 capture a first and asecond plurality of narrowband optical signals having TM and TEpolarization components, respectively. This embodiment also holds anadvantage over conventional free-space optic systems because thedispersive elements 14 and 16 can be precisely aligned to each other andto the outputs of the chip 22 during the chip manufacturing process,such that aligning the outputs of the chip 22 to the inputs of thedetector array 24 automatically aligns both of the dispersive elements14 and 16 to the detector array 24.

The detector array 24 is an array of detector elements such as, but notlimited to, surface-illuminated detector pixels or integrated waveguidephotodetectors, for example, that are arranged to receive and measurethe plurality of narrowband optical signals derived from the dispersiveelements 14 and 16 thereby measuring specific information about theinitial input optical signal 30. Typically, the detector elements arelinearly arranged to provide a linearly spaced array of detectorelements and each narrowband optical signal from the waveguide arrays 18and 20 illuminate one or more detector elements. To facilitate this, theinputs of the detector array 24 are precisely aligned with the outputsof the chip 22.

Generally, the detector array 24 shown in the various embodiments hereinis a single detector array. However, there may be some alternativeembodiments where it may be beneficial to use more than one detectorarray.

The data measured by the detector array 24 are provided to the readoutelectronics 28 via the output lines 26, which can be conductive traceson a chip or electrical wires depending on the particularimplementation. The readout electronics 28 are used to convert theelectrical signals generated (i.e. representing measured data) by thedetector array 24 into output data having a suitable format that can beused by a computing device (not shown) to analyze the output data suchas, for example, by computing an inverse Fourier transform of the outputdata. In some embodiments, the readout electronics 28 includes a FieldProgrammable Gate Array or a microcontroller that provides clock andcontrol signals to the detector array 24 in order to read the measureddata from the detector array 24 and then format the measured data usinga suitable output data format. For example, the output data format canbe a USB format so that a USB connection can be used between the readoutelectronics 28 and the computing device. In some embodiments, anotherformat such as a Camera Link or a Gigabit Ethernet connection can beused. In some embodiments, if the detector array 24 generates outputanalog signals, then the readout electronics 28 also includes a suitablenumber of analog to digital converters with a suitable number ofchannels.

It should be noted that collectively, the polarization splitter 12, andthe dispersive elements 14 and 16 can be referred to as at least twodispersive elements that are configured to receive an input opticalsignal and spatially separate the input optical signal into a firstplurality of spatially separated spectral components having a firstpolarization state and a second plurality of spatially separatedspectral components having a second polarization state over an operatingbandwidth range.

Referring now to FIG. 2, shown therein is a block diagram of anotherexample embodiment of a polarization-sensitive spectrometer 50.

The spectrometer 50 is similar to the spectrometer 10 except that thespectrometer 50 comprises an on-chip polarization splitter 52. Theon-chip polarization splitter 52 can be, but is not limited to, awaveguide polarization splitter, for example. The on-chip polarizationsplitter 52 is located on the same chip 54 as the dispersive elements 14and 16 and the waveguide arrays 18 and 20. The spectrometer 50 operatesin a similar manner as the spectrometer 10. However, the spectrometer 50is preferable to the spectrometer 10 since only the single initial inputoptical signal 30 is aligned with the chip 54 compared to both of the TMand TE input optical signals for the chip 22. In addition, the overallspectrometer 50 can be smaller and less expensive than the spectrometer10 due to the reduced number of components and the reduced alignmentrequirements.

It should be noted that collectively, the polarization splitter 52, andthe dispersive elements 14 and 16 can be referred to as at least twodispersive elements that are configured to receive an input opticalsignal and spatially separate the input optical signal into a firstplurality of spatially separated spectral components having a firstpolarization state and a second plurality of spatially separatedspectral components having a second polarization state over an operatingbandwidth range.

Referring now to FIG. 3, shown therein is a block diagram of anotherexample embodiment of a polarization-sensitive spectrometer 100. Thespectrometer 100 comprises two dispersive elements, one designed foreach polarization, that are overlaid to share a common footprint in apolarization-splitting grating configuration 102 that can provide boththe dispersion and polarization-splitting functions. Thepolarization-splitting grating configuration 102 is implemented on achip 104 along with the waveguide arrays 18 and 20. As with thespectrometer 50, only an initial input optical signal 30 is aligned withthe chip 104. This embodiment can be enabled by using apolarization-dependent effective index n_(eff) which can be engineeredin waveguide structures based on the geometry of the waveguidestructures and the materials used in the waveguide structures, as oneexample. This spectrometer 100 is preferable to the spectrometers 10 and50 since the spectrometer 100 does not require a separatepolarization-splitting component and the combined footprint of the twodispersive elements can be reduced.

It should be noted that the polarization-splitting grating configuration102 can be seen as at least two dispersive elements that are configuredto receive an input optical signal and spatially separate the inputoptical signal into a first plurality of spatially separated spectralcomponents having a first polarization state and a second plurality ofspatially separated spectral components having a second polarizationstate over an operating bandwidth range.

Referring now to FIG. 4A, shown therein is a diagram of apolarization-splitting grating configuration 150 that can provide bothdispersion and polarization-splitting functions. Thepolarization-splitting grating configuration 150 is constructed from afirst dispersive element operating for the effective index of the TEpolarization and directing the outputs to a TE output region, and asecond dispersive element operating for the effective index of the TMpolarization and directing the outputs to a TM output region. In thiscase, the two dispersive elements are configured to overlap with eachother and share a common footprint. When provided with an input opticalsignal having both TE and TM components, the wavelengths of the TMoutput optical signals do not spatially overlap with the wavelengths ofthe TE output optical signals.

An example embodiment of the polarization-splitting gratingconfiguration 150 is a PCG polarization-splitting grating 160 shown inFIG. 4B. The PCG polarization-splitting grating 160 has an output focalcurve 164. The PCG polarization-splitting grating 160 is constructedfrom a first dispersive element that has an effective index for the TEpolarization and directs the outputs to a first portion 166 of theoutput focal curve 164, and a second dispersive element that has aneffective index for the TM polarization and directs the outputs to asecond portion 168 of the output focal curve 164, wherein the twodispersive elements are configured to overlap with each other and sharea common footprint. It can be seen that there is no overlap between thefirst and second portions 166 and 168.

In other embodiments, the PCG polarization-splitting grating 160 may beconstructed from a first dispersive element that has an effective indexfor the TE polarization and directs the outputs to a first portion of anoutput of the PCG polarization-splitting grating 160, and a seconddispersive element that has an effective index for the TM polarizationand directs the outputs to a second portion of the output of the PCGpolarization-splitting grating 160, where the first and second portionsof the output may be near but not necessarily on the output focal curveof the PCG polarization-splitting grating 160.

Another example embodiment of the polarization-splitting gratingconfiguration 150 is an AWG polarization-splitting grating 170 shown inFIG. 4C. The AWG polarization-splitting grating 170 has an input freepropagation region 172, a plurality of waveguides 174 and an output freepropagation region 176 such that there is an output focal curve 178. Inthe input free propagation region 172, the light of an input opticalsignal spreads out to illuminate the plurality of waveguides 174 andtravel along different path lengths to the output free propagationregion 176 where the optical signals are focused to the outputs of theAWG polarization-splitting grating 170. The AWG polarization-splittinggrating 170 is constructed from a first dispersive element that has aneffective index of the TE polarization and directs the outputs to afirst portion 180 of the output focal curve 178, and a second dispersiveelement operating that has an effective index for the TM polarizationand directs the outputs to a second portion 182 of the output focalcurve 178, wherein the two dispersive elements are configured to overlapwith each other and share a common footprint. It can be seen that thereis no overlap between the first and second portions 180 and 182.

In other embodiments, the AWG polarization-splitting grating 170 may beconstructed from a first dispersive element that has an effective indexfor the TE polarization and directs the outputs to a first portion of anoutput of the AWG polarization-splitting grating 170, and a seconddispersive element that has an effective index for the TM polarizationand directs the outputs to a second portion of the output of the AWGpolarization-splitting grating 170, where the first and second portionsof the output may be near but not necessarily on the output focal curveof the AWG polarization-splitting grating 170.

To determine whether polarization splitting can be implemented by twodispersive elements sharing a common footprint in apolarization-splitting grating configuration, one must consider thetotal operating bandwidth of the spectrometer. The total bandwidthavailable in a spectrometer based on a polarization-splitting gratingcan be described mathematically as follows, starting with the gratingequation shown in equation 2.d(sin θ_(i)+sin θ_(m))=mλ/n _(eff)  (2)

In Equation 2, d is grating pitch, θ_(i) is input angle of an inputoptical signal, θ_(m) is output angle of an output optical signal, m ismode order, λ is wavelength of the output optical signal and n_(eff) isthe effective index (which is in general a function of wavelength). Fora central wavelength λ₀, the output angle for the TM mode with aneffective index n_(eff,TM)(λ₀) is given by equation 3:

$\begin{matrix}{\theta_{0,{TM}} = {\sin^{- 1}\left( {\frac{m\;\lambda_{0}}{d\;{n_{{eff},{TM}}\left( \lambda_{0} \right)}} - {\sin\;\theta_{i}}} \right)}} & (3)\end{matrix}$and the output angle for the TE mode with an effective indexn_(eff,TE)(λ₀) is given by equation 4.

$\begin{matrix}{\theta_{0,{TE}} = {\sin^{- 1}\left( {\frac{m\;\lambda_{0}}{d\;{n_{{eff},{TE}}\left( \lambda_{0} \right)}} - {\sin\;\theta_{i}}} \right)}} & (4)\end{matrix}$The difference between the output angles is given by equation 5:Δθ₀=θ_(0,TM)−θ_(0,TE)  (5)and can be used to provide a separation of the TE and TM polarizationcomponents at a single wavelength λ₀.

For the spectrometer 100 to operate with high fidelity over an operatingspectral range Δλ_(spec)=λ_(max)−λ_(min), it is desired that the TMminimum wavelength be separated from the TE maximum wavelength, or inother words θ_(min,TM)>θ_(max,TE), as is shown in FIGS. 4A-4C forseveral different example embodiments. These equations are derived forthe case in which n_(eff,TM)<n_(eff,TE). It should be noted that similarequations can be derived for the case in which n_(eff,TM)>n_(eff,TE).

Therefore, in this example embodiment, a TE dispersive element and a TMdispersive element are combined in a polarization-splitting gratingconfiguration exhibiting the following:

$\begin{matrix}{\theta_{\min,{TM}} > \theta_{\max,{TE}}} & (6) \\{{\sin^{- 1}\left( {\frac{m\;\lambda_{\min}}{d\;{n_{{eff},{TM}}\left( \lambda_{\min} \right)}} - {\sin\;\theta_{i}}} \right)} > {\sin^{- 1}\left( {\frac{m\;\lambda_{\max}}{d\;{n_{{eff},{TE}}\left( \lambda_{\max} \right)}} - {\sin\;\theta_{i}}} \right)}} & (7) \\{{\frac{m\;\lambda_{\min}}{d\;{n_{{eff},{TM}}\left( \lambda_{\min} \right)}} - {\sin\;\theta_{i}}} > {\frac{m\;\lambda_{\max}}{d\;{n_{{eff},{TE}}\left( \lambda_{\max} \right)}} - {\sin\;\theta_{i}}}} & (8) \\{\frac{\lambda_{\min}}{n_{{eff},{TM}}\left( \lambda_{\min} \right)} > \frac{\lambda_{\max}}{n_{{eff},{TE}}\left( \lambda_{\max} \right)}} & (9)\end{matrix}$

A polarization-splitting grating configuration can therefore be designedto operate over a desired spectral range between λ_(min) and λ_(max) byengineering appropriate values of the effective index for the TM moden_(eff,TM) and the effective index for the TE mode n_(eff,TE). This canbe done by appropriate design of the geometry and materials used in theregion of the polarization-splitting grating configuration shared by thetwo dispersive elements.

As an example, FIGS. 5A-5D show diagrams illustrating the birefrengenceversus wavelength and bandwidth as well as the relation betweenbandwidth and wavelength for a polarization-sensitive spectrometer thatuses an example embodiment of a PCG polarization-splitting grating toprovide both the dispersion and polarization-splitting functions. Inparticular, FIGS. 5A-5B show the calculated birefringence for a sharedpropagation region consisting of a 150 nm thick slab of silicon nitridesurrounded by silicon dioxide in a slab waveguide PCG device. FIGS.5C-5D show a 46 nm bandwidth that is achievable while completelysplitting the polarizations for this same slab waveguide PCG device.These results were obtained by using modal effective index simulationsperformed using a Finite Element Method (FEM) mode solver. The bandwidthcan be raised or lowered by altering the geometry and the materialrefractive indices of the slab waveguide PCG device. For example, insome embodiments, the waveguide core, bottom cladding, and top claddingcan be comprised of various materials such as, but not limited tosilicon nitride, silicon oxide, silicon oxynitride, silicon-rich siliconnitride, silicon, SU8, doped glass, other polymers, or other suitablematerials, for example. In general, more contrast in effective indicesfor the TE and TM polarization components leads to a larger amount ofsplitting of the TE and TM polarization components and a largerachievable bandwidth for the polarization-splitting gratingconfiguration and the spectrometer system.

In a more general case, the TE and TM dispersive elements could also beoperating at different grating mode orders m_(TE) and m_(TM). In thatcase, the polarization-splitting grating can be designed to satisfy thecondition shown in equation 10.

$\begin{matrix}{\frac{m_{TM}\lambda_{\min}}{n_{{eff},{TM}}\left( \lambda_{\min} \right)} > \frac{m_{TE}\lambda_{\max}}{n_{{eff},{TE}}\left( \lambda_{\max} \right)}} & (10)\end{matrix}$

Referring now to FIG. 6, it should be noted that there may also be casesin which the bandwidth of the input optical signal to apolarization-splitting grating configuration 190 is larger than itsmaximum bandwidth, in which case the output polarization signals are notcompletely separated and some of the center-most outputs will contain aTE narrowband optical signal at one wavelength and a TM narrowbandoptical signal at another wavelength. In other words, in these cases,there will be an overlap in wavelength in the TE and TM outputs of thepolarization-splitting grating configuration 190. This overlap is due tothe fact that the TE and TM outputs at these wavelengths arrived in thesame output of the polarization-splitting grating configuration 190because they experienced the same effective index n_(eff) within thepolarization-splitting grating configuration 190. In this case, asecondary splitting element 192 based on either polarization orwavelength may be used to completely separate the TE and TM outputsignals. While this adds complexity to the spectrometer, it could beused if the maximum splitting bandwidth (as calculated above) isinsufficient for a given application.

As previously mentioned, it is also possible to use apolarization-splitting grating configuration to independently focus thetwo polarization components of an input optical signal in addition tosplitting them. To accomplish this, the polarization-splitting gratingconfiguration can be designed by requiring the grating facets, numberedby integers i, to have center locations (x_(i), y_(i)) that satisfy thetwo constraint conditions shown in equations 11 and 12.ε_(1,i)(x _(i) ,y _(i))=0; ε_(2,i)(x _(i) ,y _(i))=0  (11, 12)

An example of a constraint function that can be used to create a gratingwith a particular stigmatic or aberration-free output point for awavelength λ₀ is shown in equation 13.

$\begin{matrix}{{ɛ_{1,i}\left( {x,y} \right)} = {r_{1} + r_{2} + \frac{i\; m\;\lambda_{0}}{n_{eff}} - r_{1,0} - r_{2,0}}} & (13)\end{matrix}$

In equation 13 i is an integer, r₁ is the distance between the inputpoint (α₁, b₁) and an arbitrary point (x, y), r₂ is the distance betweenthe output point (a₂, b₂) and (x, y), r_(1,0) is the distance between(a₁, b₁) and the grating pole (x₀, y₀), and r_(2,0) is the distancebetween (x₀, y₀) and (a₂, b₂). In other words, a constraint function ofthis form guarantees that the grating facets are placed to provideaberration-free focusing of the wavelength λ₀ onto the output point (a₂,b₂). Specifically, each facet is positioned to provide an optical pathlength difference of mλ₀/n_(eff) along the optical path from the inputto the facet to the output, compared to adjacent facets.

The two constraint functions can be used to generate two separatestigmatic output points for the central wavelength λ₀ for eachpolarization component according to equation 14 for the TM outputlocated at position (a₂,b₂)_(TM) and equation 15 for the TE outputlocated at position (a₂,b₂)_(TE).

$\begin{matrix}{{ɛ_{1,i}\left( {x,y} \right)} = {r_{1} + r_{2,{TM}} + \frac{i\; m\;\lambda_{0}}{n_{{eff},{TM}}} - r_{1,0} - r_{2,0,{TM}}}} & (14) \\{{ɛ_{2,i}\left( {x,y} \right)} = {r_{1} + r_{2,{TE}} + \frac{i\; m\;\lambda_{0}}{n_{{eff},{TE}}} - r_{1,0} - r_{2,0,{TE}}}} & (15)\end{matrix}$In other words, aberration-free focusing of both polarizations isprovided by solving for facet positions that simultaneously satisfy bothequations for the TE dispersive element and the TM dispersive element.Each facet is positioned to simultaneously provide an optical pathlength difference of m_(TE)λ₀/n_(eff,TE) and m_(TM)λ₀/n_(eff,TM)compared to adjacent facets. By combining engineeredpolarization-splitting with engineered polarization focusing, by usingthe equations shown herein for example, one can create a spectrometerwhich separates and measures the two polarization components of an inputoptical signal with high performance and high efficiency.

Referring now to FIG. 7A, shown therein is an example embodiment 200 ofan arrangement between a chip 202 containing one or more dispersiveelements and a second, different chip 204 containing a detector array.In this case there is an edge-to-face orientation between the dispersiveelement chip 202 and the detector array chip 204 in order to transmitoutput optical signals from the dispersive element chip 202 to the planecontaining the inputs of the detector array 204. This orientation issuitable for a detector array that has a 1D (i.e. linear) array ofdetector elements.

Referring now to FIG. 7B, shown therein is another example embodiment250 of an arrangement between a chip 252 containing one or moredispersive elements and a second, different chip 254 containing adetector array. In this case there is a face-to-face orientation betweenthe dispersive element chip 252 and the detector array chip 254 wherethe dispersive element chip 252 contains an element or elements todirect light out of the plane of the first chip and to the planecontaining the inputs of the detector array 254. Examples of suchelements include, but are not limited to, grating couplers, 45° mirrors,or directional scattering elements. This orientation is suitable for adetector array that has a either a 1D (i.e. linear) array of detectorelements or a 2D (i.e. area) array of detector elements.

It should be noted that in both FIGS. 7A-7B, the dispersive element chip202/252 contains optical devices in the plane of the chip, and thedetector array chip 204/254 receives light normal to its chip surface.

Referring now to FIG. 8, shown therein is another example embodiment ofa spectrometer 300 having a splitting element 302, multiple dispersiveelements 304 to 308 and multiple waveguides 18, 310 and 20 on a planarsubstrate 312 (i.e. chip or IC) and a single detector array 24. Thespectrometer 300 also comprises readout electronics that are coupled tothe detector array 24. In alternative embodiments, at least one of thedetector array 24 and the splitting element 302 may be implemented onthe planar substrate 312. In alternative embodiments, the waveguides 18,310 and 20 may not be needed.

The components of the spectrometer 300 function in the same way assimilar components described previously. However, in this embodiment,there is more than one input signal that is provided to the spectrometerand the dispersive elements collectively generate more than twopluralities of spatially separated spectral components that do notnecessary have different polarizations with respect to one another.Furthermore, it should be noted in this general case that the inputoptical signal can be split based on a principle other than polarizationsuch as simply splitting the input optical signal into two separate,even or uneven strength input optical signals.

In use, a first initial input optical signal (Optical input #1) isreceived by the splitting element and separated into first and secondinput optical signals. The splitting may be done based on polarization,wavelength, amplitude or some other characteristic of the first initialinput optical signal. The first and second input optical signals aresent to the first and second dispersive elements 304 and 306respectively. The second initial input optical signal (Optical input #2)is sent directly to the third dispersive element 308.

The dispersive elements 304 to 308 each produce a plurality of spatiallyseparated spectral components that are each captured as a plurality ofnarrowband optical signals by the corresponding waveguides 18, 310 and20. The three pluralities of narrowband optical signals are thenprovided to different regions of the detector array 24 to measure thepluralities of narrowband optical signals. The measurements are thensent to the readout electronics 28 via electrical connections 26 wherethe measurements are read and provided as output data for viewing orother purposes.

The spectrometer 300 represents a general case in which it may beadvantageous to combine multiple (e.g. two or more) dispersive elementsalong with zero, one, or more splitters and a single detector array intoa multi-functional spectrometer. Such embodiments provide manyadvantages over conventional schemes in which each dispersive elementwould have its own detector array. Examples of these advantages include,but are not limited to, reduction in size, reduction in cost, reductionin computing requirements, simultaneous triggering of all outputs (sincea single detector array is performing the detection), and a reduction incomplexity of temperature dependency since the temperature dependency ofonly a single detector array need be taken into account compared totaking into account the different temperature dependencies of two ormore separate detector arrays in conventional schemes. An example ofwhere a single detector array can be shared for different dispersiveelements is when there is a plurality of pixels in the detector array,such as 2048 pixels, but only a few pixels are needed to measure theoutput of each of several different dispersive elements.

It should be noted that the spectrometer 300 may be used in situationswhere multiple optical input signals may enter a spectrometer. Forexample, the different input optical signals may carry optical signalswith different wavelength ranges or different polarization states, orthe optical signals may be captured from different samples or differentlocations on the same sample, or the or the optical signals may bereceived from multiple outputs of an interferometer, for example.

It should also be noted that in various alternative embodiments of thespectrometer 300, one or more splitting elements may be included tosplit one input optical signal into multiple input optical signals thatmay differ in at least one of wavelength, polarization state, or otherproperties of the input optical signal.

It should also be noted that in various embodiments of the spectrometer300, the different dispersive elements on the chip 312 can be configuredto operate over different wavelength ranges, to disperse input opticalsignals into different resolutions, to operate for differentpolarization states, or to operate differently with respect to otheroptical properties of the input optical signals.

The various example embodiments that in accordance with the teachingsherein show that a chip-based system allows for many complexpermutations of optical elements to be implemented for a spectrometerwhich would be impossible or impractical to implement in free-spaceoptics.

In an alternative for the various spectrometer embodiments describedherein, the detector array (and/or readout electronics) can be locatedon the same chip as the spectrometer in order to completely eliminateany manual alignment of the outputs of the spectrometer to the inputs ofthe detector array.

In a further alternative for the various spectrometer embodimentsdescribed herein, if the detector array is located on a separate chip asis shown in FIGS. 1 to 3, 7A, 7B and 8, then one or more lenses can beplaced between the dispersive element chip and the detector array torefocus the output optical signals from the dispersive element chip andto improve coupling efficiency.

In one aspect, the various polarization-sensitive spectrometerembodiments illustrated and described herein in which at least onedispersive element configuration is implemented on an integrated orplanar substrate allow for the construction of smaller, simpler,single-camera or single detector array polarization-sensitivespectrometers with fewer components requiring manual alignment. Thisreduces costs as well as improves reliability and robustness compared toconventional spectrometers that are implemented using free-space opticalelements and are bulky and expensive. In particular, the solid-stateform of the various polarization-sensitive spectrometer embodimentsdescribed herein provides increased robustness which is unequaled incomplex free-space optics systems and also enables point-of-careapplications, meaning that at least some of the spectrometer systemsdescribed herein can be taken in the field to analyze various samples,which has previously not been possible. The elements of the variouspolarization-sensitive spectrometer embodiments illustrated herein thatare implemented on an integrated substrate can also be easily integratedwith other on-chip components.

In another aspect, at least some of the various polarization-sensitivespectrometer embodiments illustrated and described herein can reducespeckle noise in OCT images. This is done by taking advantage of thedifference in polarization characteristics between a speckle and lightfrom the target. In addition, this technique has value in discriminatingbetween a desired (predominantly polarized) OCT signal from apredominantly unpolarized background signal (for example,singly-scattered vs. multiply-scattered light).

In another aspect, at least some of the various polarization-sensitivespectrometer embodiments illustrated and described herein can be used toremove common-mode noise that affects both polarizations in an opticalsignal. A conventional spectrometer that is insensitive to thepolarization of the incoming light cannot differentiate common-modenoise in an acquired spectrum. But in the case of apolarization-sensitive spectrometer according to the teachings herein,it is possible to implement more sophisticated discriminatory criteriain order to filter common-mode noise. For example, imposing smoothnesslimits on the changing polarization with wavelength, acquiring thetarget phase image, and proper phase calibration can all be used tomitigate the effects of common-mode noise, which is beyond thecapabilities of single-polarization or polarization-insensitivespectrometers.

In another aspect, the various polarization-sensitive spectrometerembodiments illustrated and described herein eliminate the need forzero-birefringence (or “polarization compensation”) in waveguidedevices.

It should be noted that while the various embodiments described hereinhave described processing an input optical signal to derive a first anda second plurality of narrowband optical signals that have TE and TMpolarization states respectively, it should be understood that theseembodiments can be designed to more generally process the input opticalsignal to derive a first and a second plurality of narrowband opticalsignals having a first polarization state and a second polarizationstate, respectively. Accordingly, the usage of various parameters forthe TE and TM modes can more generally be represented by parameters forfirst and second polarization states.

It should be noted that in the various embodiments according to theteachings herein, two or more pluralities of narrowband optical signalsmay be derived from two or more pluralities of spatially separatedspectral components in several different ways. For example, in someembodiments, this derivation may be performed by two or more waveguidearrays that capture two or more pluralities of narrowband opticalsignals from two or more pluralities of spatially separated spectralcomponents that are provided as inputs to the two or more waveguidearrays. Alternatively in some embodiments, this derivation may beperformed by a single detector array that captures two or morepluralities of narrowband optical signals from two or more pluralitiesof spatially separated spectral components that are provided as inputsto the single detector array.

At least some of the elements of the various OCT embodiments describedherein, may at least partially be implemented via software and writtenin a high-level procedural language such as object oriented programming,a scripting language, assembly language, machine language, firmware orany other suitable programming language as needed. The program code canbe stored on a storage media or on a computer readable medium that isreadable by general or special purpose programmable electronics having aprocessor or associated hardware that is sufficient to implement therequired functionality. The program code, when read by a processor orassociated hardware, configures these elements to operate in a specificand predefined manner in order to perform at least one of the functionsdescribed herein.

While the above description provides examples of various embodiments, itwill be appreciated that some features and/or functions of the describedembodiments are susceptible to modification without departing from theprinciples of operation of the described embodiments. Accordingly, whathas been described above has been intended to be illustrative of thesubject matter described herein and non-limiting and it will beunderstood by persons skilled in the art that other variants andmodifications may be made without departing from the scope of theclaimed subject matter as defined in the claims appended hereto.Furthermore, the scope of the claims should not be limited by thepreferred embodiments and examples described herein, but should be giventhe broadest possible interpretation that is consistent with thedescription as a whole.

The invention claimed is:
 1. A spectrometer comprising: at least twodispersive elements, each configured to receive an input optical signaland to generate a plurality of spatially separated spectral components,at least a portion of the at least two dispersive elements beingimplemented on a first integrated substrate; a single detector arraycoupled to the at least two dispersive elements and configured toreceive and to measure two or more of the pluralities of spatiallyseparated spectral components; and at least one waveguide implemented onthe first integrated substrate for providing light coupling between theat least two dispersive elements and the single detector array, whereinthe at least one waveguide comprises at least one slab waveguide or atleast one waveguide array.
 2. The spectrometer of claim 1, wherein thespectrometer comprises the at least one waveguide array implemented onthe first integrated substrate for providing light coupling between theat least two dispersive elements and the single detector array, the atleast one waveguide array producing one or more pluralities ofnarrowband optical signals from one or more of the pluralities ofspatially separated spectral components.
 3. The spectrometer of claim 1,further comprising at least one splitting element that is configured toreceive an input optical signal of the spectrometer and to produce atleast one input optical signal for each of the at least two dispersiveelements.
 4. The spectrometer of claim 3, wherein the at least onesplitting element is implemented on the first integrated substrate or aseparate integrated substrate.
 5. The spectrometer of claim 3, whereinthe at least one splitting element is configured to perform splittingbased on wavelength and to produce the at least one input optical signalfor each of the at least two dispersive elements with differentwavelength ranges that are each a subset of the wavelength range of theinput optical signal of the spectrometer.
 6. The spectrometer of claim3, wherein the at least one splitting element is configured to separatethe input optical signal of the spectrometer into the at least one inputoptical signal for each of the at least two dispersive elements withdifferent amplitudes that are smaller than an amplitude of the inputoptical signal of the spectrometer.
 7. The spectrometer of claim 3,wherein the at least one splitting element is configured to performsplitting based on polarization and to produce the at least two inputoptical signals with different polarization states from one another. 8.The spectrometer if claim 7, wherein the at least one splitting elementcomprises one of a planar concave grating element and an arrayedwaveguide grating element.
 9. The spectrometer of claim 1, wherein thespectrometer is configured to receive two or more input optical signalsfor coupling with the at least two dispersive elements without splittingthe input optical signals.
 10. The spectrometer of claim 1, wherein oneor more of the dispersive elements comprises a planar concave grating oran arrayed waveguide grating.
 11. The spectrometer of claim 1, whereinthe at least two dispersive elements are configured to share a commonfootprint in a polarization-splitting grating configuration implementedon the first integrated substrate and having a difference in effectiveindices of refraction for first and second polarization states over anoperating bandwidth range to spatially separate two polarizationcomponents of the initial input optical signal such that a minimumoutput angle of a first plurality of spatially separated spectralcomponents having a first polarization state is larger than a maximumoutput angle of a second plurality of spatially separated spectralcomponents having a second polarization state over the operatingbandwidth range.
 12. The spectrometer of claim 1, further comprisingreadout electronics to receive the measurements from the single detectorarray and to generate output samples thereof having a desired outputdata format.
 13. The spectrometer of claim 12, wherein the singledetector array and the readout electronics are implemented on the sameintegrated substrate as the dispersive elements, or on a differentintegrated substrate.
 14. The spectrometer of claim 1, wherein thesingle detector array is on a second integrated substrate of a differentchip and a face or an edge of the first integrated substrate is directedto a face of the second integrated substrate in order to capture the twoor more pluralities of spatially separated components at inputs of thesingle detector array.
 15. The spectrometer of claim 1, wherein thesingle detector array is located on a different integrated substratethan the first integrated substrate and one or more lenses are locatedbetween the first integrated substrate and the single detector array torefocus the two or more pluralities of narrowband optical signals and toimprove coupling efficiency.
 16. The spectrometer of claim 1, whereinthe single detector array is a linear detector array.
 17. A method ofmeasuring two or more pluralities of spatially separated spectralcomponents, the method comprising: receiving at least one input opticalsignal; generating two or more pluralities of spatially separatedspectral components from the at least one input optical signal byproviding the at least one input optical signal to at least twodispersive elements with at least a portion of the at least twodispersive elements being implemented on a first integrated substrate;and measuring the two or more pluralities of spatially separatedspectral components with a single detector array, wherein at least onewaveguide is implemented on the first integrated substrate to providelight coupling between the at least two dispersive elements and thesingle detector array, the at least one waveguide comprising at leastone slab waveguide or at least one waveguide array.
 18. The method ofclaim 17, wherein at least one splitting element is implemented on thefirst integrated substrate or a separate integrated substrate and isconfigured to receive an input optical signal of the spectrometer and toproduce at least one input optical signal for each of the at least twodispersive elements.
 19. The method of claim 18, wherein the splittingis performed based on one of wavelength, polarization, and amplitude.20. The method of claim 18, wherein the splitting is performed using oneof a planar concave grating element or an arrayed waveguide gratingelement.
 21. The method of claim 17, wherein the at least two dispersiveelements comprise first and second dispersive elements and the methodfurther comprises: receiving a first input optical signal at the firstdispersive element disposed on the first substrate; generating a firstplurality of spatially separated spectral components with the firstdispersive element; receiving a second input optical signal at thesecond dispersive element disposed on the first substrate; andgenerating a second plurality of spatially separated spectral componentswith the second dispersive element.
 22. The method of claim 17, whereinthe method further comprises: receiving an initial input optical signalas the at least one input optical signal at a polarization splitter;generating the first input optical signal to have a first polarizationstate over an operating bandwidth range using the polarization splitter;generating the second input optical signal to have the secondpolarization state over the operating bandwidth range using thepolarization splitter; generating the first plurality of spatiallyseparated spectral components having the first polarization state withthe first dispersive element; and generating the second plurality ofspatially separated spectral components having the second polarizationstate with the second dispersive element.
 23. The method of claim 17,wherein the at least two dispersive elements are configured to share acommon footprint in a polarization-splitting grating configurationimplemented on the first substrate, the polarization-splitting gratingconfiguration having a difference in effective indices of refraction forfirst and second polarization states over an operating bandwidth rangeand the method comprises: receiving an initial input optical signal asthe at least one input optical signal at the polarization-splittinggrating configuration; and spatially separating the initial inputoptical signal into a first plurality of spatially separated signalshaving a first polarization state and a second plurality of spatiallyseparated signals having a second polarization state.
 24. The method ofclaim 23, wherein the method comprises: spatially separating the twopolarization states of the initial input optical signal such that aminimum output angle of the first plurality of spatially separatedspectral components having the first polarization state is larger than amaximum output angle of the second plurality of spatially separatedspectral components having the second polarization state over theoperating bandwidth range.
 25. The method of claim 24, wherein thepolarization-splitting grating configuration has a difference ineffective indices of refraction for the first and second polarizationstates such that the first plurality of spatially separated spectralcomponents having the first polarization state is disposed along a firstportion of an output of the polarization-splitting grating configurationand the second plurality of spatially separated spectral componentshaving the second polarization state is disposed along a second portionof the output different than the first portion.